Ultrahigh molecular weight polyethylene (hereinafter referred to as “UHMWPE”) is commonly used to make prosthetic joints such as artificial hip joints. A conventional way of making implants is to machine them out of extruded UHMWPE or block molded UHMWPE. Alternatively, the implants are directly molded from UHMWPE. The implants are then packaged and next sterilized with radiation, gas plasma, or ethylene oxide. The sterilized packaged implants are then sold and the implants are removed from their packages when the surgeons are ready to place them in the patients' bodies.
In recent years, it has become increasingly apparent that wear of acetabular cups of UHMWPE in artificial hip joints introduces many microscopic wear particles into the surrounding tissues. The reaction to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the prosthesis is anchored. Eventually, the prosthesis becomes painfully loose and must be replaced.
Improving the wear resistance of the UHMWPE socket and, thereby, reducing the rate of production of wear debris would extend the useful life of artificial joints and permit them to be used successfully in younger patients. Consequently, numerous modifications in physical properties of UHMWPE have been proposed to improve its wear resistance.
UHMWPE components are also known to undergo a spontaneous, post-fabrication increase in crystallinity and changes in other physical properties. {see e.g., Rimnac, C. M., et al., J. Bone & Joint Surgery, 76-A(7): 1052-1056 (1994)}. These changes occur even in stored (non-implanted) prostheses after sterilization with gamma radiation (“gamma radiation sterilization”) which is typically used for sterilization irradiation. The industrial standard for the gamma sterilization dose is between 2.5 Mrad to 4 Mrad. Typically, 3 to 3.5 Mrad is used.
Gamma radiation initiates an ongoing process of chain scission, crosslinking, and oxidation or peroxidation involving the free radicals formed by the irradiation. These degradative changes may be accelerated by oxidative attack from the joint fluid bathing the implant. Oxidation due to post-irradiation aging of UHMWPE has been associated with pitting, delamination and fracture in total hip replacements. (Walsh, H., et al., “Factors that Determine the Oxidation Resistance of Molded 1900: Is it the Resin or the Molding”, Poster Session—Polyethylene, 46th Annual Meeting, Orthopaedic Res. Soc., Mar. 12-15, 2000, Orlando, Fla., U.S.A., page 543, first sentence. Hereinafter referred to as “Walsh p. 543, supra”). Additionally, many studies have shown that oxidation due to open-air gamma-irradiation sterilization of UHMWPE components is associated with delamination and fracture in artificial knee-joints. {Mori, A., et al., “Mechanical Behavior of UHMWPE When Mixed with Vitamin E”, Nakashima Medical Division, Nakashima Propeller Co., Ltd., hand-out at the 47th Ann. Mtg., Orthopaedic Res. Soc., Feb. 25-28, 2001, San Francisco, Calif., first sentence. Hereinafter referred to as “Mori Hand-Out”.} In another publication, Mori et al. indicate that “Apparently, it is widely recognized through numerous research projects that the largest cause of wear on [UHMWPE], used as the bearing surface in artificial joints, is the oxidative degradation following γ-ray irradiated sterilization in air.” (Mori et al., “Effects of Manufacturing Method and Condition on UHMWPE Wear”, Society for Biomaterials, Sixth World Biomaterials Congress Transactions, p 1122, year 2000, first sentence. Hereinafter referred to as “Mori p. 1122, supra”).
Consequently, several companies have modified the method of gamma radiation sterilization to improve the oxidation-resistance and thus the wear resistance of the UHMWPE components. For example, in the case of UHMWPE acetabular cups, this has typically involved packaging the cups either in an inert gas (e.g., Stryker-Osteonics-Howmedica, Inc., Rutherford, N.J., USA), in a partial vacuum (e.g., Johnson & Johnson, Inc., New Brunswick, N.J., USA) or with an oxygen scavenger (e.g., Sulzer, Inc., Winterthur, Switzerland) during gamma radiation sterilization. Alternatively, gas plasma or ethylene oxide are used to sterilize the implant in order to avoid the free radicals generated by irradiation sterilization.
Further, in recent years, there have been reports that some UHMWPE may be particularly resistant to oxidation. Starting from the 1970's, Zimmer Inc. (Warsaw, Ind., USA) has been making an UHMWPE commonly known in the industry as “net-shaped molded UHMWPE” (also known as “direct-molded UHMWPE”).
Walsh, H., et al., reported that “components made by directly molding 1900 resin are resistant to post irradiation aging for periods up to 10 years. Further, these directly molded devices have excellent clinical track records and exhibit extremely low incidences of pitting, delamination and fracture”, and that “components made by direct molding of 1900 resin have a unique resistance to oxidation. Machined components from extruded 1900 bar or compression molded 1900 sheet do not exhibit the same oxidation resistance. It is also clear from Table 1 that extruded bar and compression molded sheet of 4150 resin are also susceptible to post irradiation aging . . . Table 2 shows that directly molding either 4150 or 1990 resin provides an oxidation-resistant material. As the resin used in the directly molded 4150 was the same lot as that used in the extruded bars, the only difference between the samples was the manufacturing method.” (Left col., first and second sentences, and right col., second and third paragraphs of Walsh p. 543, supra).
Since the rate of oxidation is slow, and typically takes 5 years of post-irradiation aging (i.e., 5 years of shelf aging) for density values to rise above 0.95 g/cc, an indicator of oxidation, Walsh et al reported an accelerated aging protocol to mimic 5 years shelf aging of UHMWPE. (Walsh, H. A., et al., “A True, Reproducible Accelerated Aging Protocol To Mimic 5 Year Shelf Aging of UHMWPE”, Poster Session—Polyethylene, 46th Ann. Mtg., Orthopaedic Res. Soc., Mar. 12-15, 2000, Orlando, Fla., U.S.A., page 542. Hereinafter referred to as “Walsh p. 542, supra”).
Walsh p. 542, supra, reported that specimens directly molded from 4150 HP powder to provide a modulus of 600-800 MPa did not oxidize, when aged with its accelerated aging protocols, as compared to HSS/PolyHi Solidur/Ticona reference extruded 4150 HP, which was machined into specimens.
On the other hand, Mori p. 1122, supra, compared specimens: (1) machined from GUR 4150 HP ram extruded bar stock; (2) custom molded directly from GUR1150 resin and using compression pressure of 45 kgf/cm2; and (3) custom molded directly from GUR1150 resin and using compression pressure of 200 kgf/cm2. Mori et al., observed that directly compression molded specimens with 45 kgf/cm2 compression pressure and ram extruded specimens showed highly-developed oxidation compared to the 200 kgf/cm2 directly compression molded specimen. Thus, Mori et al., reported that: “It is generally said that direct compression molded UHMWPE provide an unexpected resistance to oxidation. However, the results obtained from this study reveal that granted directly compression molding method was used, unsuitable conditions of the process could cause less resistance to oxidation.” (Mori p. 1122, supra, first paragraph under the “Discussion” section).
In another paper, Mori, A., et al., compared two groups of UHMWPE specimens made from: (1) directly molded from UHMWPE powder (GUR1020, Ticona) at 10 MPa, and (2) directly molded from a mixture of GUR1020 powder and Vitamin E, also at 10 MPa. Mori et al., then gamma sterilized in air (at 25 kGy which is equivalent to 2.5 Mrad) and then applied accelerated aging techniques to both groups. Mori et al reported that the addition of Vitamin E to UHMWPE powder provided a unique resistance to oxidation, and preserved the initial mechanical properties. (Mori et al., “Mechanical Behavior of UHMWPE When Mixed with Vitamin E”, Poster Session--Polyethylene, 47th Ann. Mtg., Orthopaedic Res. Soc., Feb. 25-28, 2001, San Francisco, Calif., p. 1017. Hereinafter referred to as “Mori p. 1017, supra”). N. Tomita, a co-author of Mori p. 1017, supra, has previously reported that his gamma-irradiated vitamin-E-containing specimens demonstrated no subsurface crack formation and no flaking-like destruction. {Tomita, N., et al., “Prevention of Fatigue Cracks in Ultrahigh Molecular Weight Polyethylene Joint Components by the Addition of Vitamin E”, J. Biomed Mater Res (Appl Biomater) 48: 474-478 (1999)}.
Mori et al teach that: “Recently the practice of crosslinking UHMWPE after gamma-ray or electron-beam irradiation has developed to improve resistance to oxidation. However, the process of crosslinking of UHMWPE is complex, and it is difficult to remove the free radicals created by irradiation. It is suggested that the addition of harmless Vitamin E is a very simple and extremely effective method to prevent oxidation degradation and maintain the stability of UHMWPE component in artificial joints.” (Mori 1017, supra, paragraph bridging left and right columns. See also, Mori hand-out, second and third last sentences).